(a) Technical Field
The present invention relates to a method for controlling engine torque in a hybrid vehicle. More particularly, the present invention relates to a method for controlling engine torque in a hybrid vehicle, in which a point of time when an intake cam is activated is determined such that an engine demand torque is controlled to a level at which system efficiency is maximized within a limited engine output range before activation of the intake cam and the engine demand torque is controlled to a level at which the system efficiency is maximized after activation of the intake cam.
(b) Background Art
Generally, a hybrid vehicle can be defined as a vehicle that employs an electric motor as an auxiliary power source as well as an internal combustion engine to provide a reduction in exhaust gas and an improvement in fuel efficiency. Typically, a hybrid vehicle includes an electric vehicle (EV) mode, which is a pure electric vehicle mode using only the power of the motor, a hybrid electric vehicle (HEV) mode, which is an auxiliary mode using the rotational force of the motor as an auxiliary power source with the use of the rotational force of the engine as a main power source, and a regenerative braking (RB) mode, in which braking energy or inertia energy of the vehicle produced by braking or during driving by inertia is suitably recovered by power generation of the motor and charged in a battery.
In the hybrid vehicle, as the EV mode and the HEV mode are repeatedly switched, and the engine is frequently turned on and off during driving. The engine in a hard HEV is more frequently turned on and off during driving than the engine in a soft HEV.
In particular examples, for example in the case of the hard HEV, the number of times when the engine is turned on and off during driving, in accordance with the federal test procedure 75 (FTP-75), which is a test mode for measuring fuel efficiency, is more than several times.
In the hard HEV, during switching from the EV mode, in which the engine is turned off, to the HEV mode, no impact may be applied to the vehicle if torque input to a transmission is constant. Accordingly, for this purpose, it is necessary to reduce the motor torque and increase the engine torque to make the transmission input torque constant as shown in the exemplary torque diagram of FIG. 1.
In the hard HEV, during the shift to the HEV mode after the vehicle is driven in the EV mode, a mode during which the engine is turned off, it takes a predetermined amount of time until the oil pressure in the engine reaches a predetermined level that is similar to that of a gasoline vehicle, which occurs repeatedly under the condition where the engine is turned off. Accordingly, during shifting from the EV mode to the HEV mode, in order to satisfy driver demand torque, i.e., in order to make the transmission input torque constant, a high engine torque is required.
Accordingly, suitably high engine torque is required to allow the operating condition of the HEV mode to be at maximum system efficiency. Accordingly, in the HEV mode, the engine generates a high torque, and the motor charges a main battery, in preparation for the operations of electrical loads and the next EV mode condition.
During switching from the HE mode, in which the engine is turned off, to the HEV mode, the engine oil pressure is not activated for a period of within several seconds, and the operation of the intake cam included in an intake variable valve timing mechanism is considerably unlikely, and thus the engine generates less than 70% of the maximum torque that the engine actually has.
For example, as shown in the exemplary torque diagram of FIG. 2, since the intake cam included in the intake variable valve timing mechanism of the engine is not operated before the engine oil pressure reaches a predetermined level, the engine torque is temporarily insufficient, and thus the transmission input torque is not maintained suitably constant during switching from the EV mode to the HEV mode.
Accordingly, as the transmission input torque is not maintained constant temporarily, an impact or vibration is applied to the vehicle.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.